Added link to php-ga-measurement-protocol

Hi @thomasbachem,

I hope you would consider placing this link to the php-ga-measurement-protocol library. I'm guessing your project still gets a lot of traffic since it was the only proper way of doing serverside tracking with GA and I think this link can help people navigate to a proper PHP library for the Measurement Protocol API. 

I'll add a thank you and link in the readme of php-ga-measurement-protocol.

Thank you